Mechanistic modeling of coupled ammonium and nitrate uptake by onions using the finite element method

A two-dimensional finite element model for predicting N uptake by onion (Allium cepa L. cv. 'Norstar 210B') was developed using variable space intervals in zones of high ionic gradient near the root surface and flexible grid size describing shrinkable subdomains of increasing root competition. The rationale included assumptions of the Cushman-Barber model, the mobile layer concept, the type of elements, and the discrete variables of the system. The model can predict simultaneous NO3- and NH4+ uptake by coupling a nitrification model and plant-specific Michaelis-Menten kinetic paremeters. The model output was compared with measured N uptake based on ammonium sulfate and ammoniated peat (AP) applied to four soils (a silt loam, two sandy loams, and a sand) in a growth chamber. The observed N uptake was close to the predicted in all soils but one, where NH4+ uptake capacity was probably attained. Root growth rate was a sensitive parameter reducing N uptake from AS compared with AP. The S-shaped uptake curves were obtained for the silt loam and sandy loam soils receiving AP. Ammonium profiles were steep. Nitrate profiles were smoother. Nitrate was the dominant N form taken up even when NH4+ was the major available form. Total N uptake was smallest in the sand, which had a low moisture content and maintained a high NH4+ gradient, and the highest in the silt loam, which showed the smallest buffering capacity and the largest volumetric water content, both known to increase NH4+ and NO3- diffusivity.